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SINGAPORE — SMRT said yesterday it is putting its longer-term track upgrade and renewal projects on the backburner, in order to focus on preventing a recurrence of Tuesday’s unprecedented train service outage, starting with finding out what exactly went wrong that day.

With a third day of checks still failing to pinpoint what caused the series of power trips preceding the 3.5-hour shutdown of the North-South and East-West lines — which are the two oldest lines on the network — the operator said it has asked some retired staff with experience in network power issues to help it investigate.

As investigations continue, SMRT is working with external experts to review how robust the design of the lines’ power network is and to find ways to segment it further such that the network-wide power failure seen on Tuesday will not happen again. SMRT has also brought forward the procurement of additional condition-monitoring systems developed jointly with research and development agencies.

In a statement yesterday, Mr Koh Yong Guan, who chairs SMRT’s board of directors, apologised for the disruption and said full support is being given to the operator’s management and staff to establish the cause of the incident, as well as to prevent a recurrence.

“It is of great concern to us that so many commuters were affected by this disruption during the evening peak hours when many were making their way back home. In particular, we apologise to our Muslim commuters who were returning home to break fast,” said Mr Koh.

SMRT also acknowledged that there were gaps in how it handled the shutdown “because of the expansive network-wide disruption”. It said: “Bus bridging could not be activated as all available and additional bus resources were already deployed to augment the free bus services along the route of the network ... We note all the feedback received from affected commuters and will continue to strive to improve our service delivery.”

The shortage of alternative transport modes was a bugbear for many of the commuters stranded by the breakdown, who struggled to get onto packed buses or find taxis.

Giving an update of its investigations, SMRT said it has completed checks on all equipment that could have contributed to the disruption.

“SMRT has assessed that it is highly unlikely that the disruption could have been caused by a defective train,” it said. “We are actively continuing our investigation into the possible degradation in the insulation of power substation, power cable or third rail.”

Initially, a defective train was suspected to have triggered the power trips. Checks done on the same night of the disruption surfaced other problems. Testing and investigations will continue over the weekend, with the full cycle of checks to be repeated, said SMRT. Where possible, it will also try to re-enact the conditions under which the series of power trips happened to try to find the cause.

On Thursday, the Land Transport Authority said it would separately hire an independent consultant specialising in transit power systems to assess the overall condition of the entire power supply infrastructure and other railway trackside installations for the two affected lines. The assessment could start this month once the experts are on board, the regulator had said.